I was psyched when Sigma sent me an 18-250mm lens to test, because I am lazy and love the idea of a single lens that I can use to take most shots. At $US530, it seemed like a decent price for that much range. But then I got to thinking...
It's still a good price for all that it does. It's not a super fast lens—a decent F3.5-6.3—but it has an optical image stabiliser, which does wonders at the outer zoom ranges. The only other 18-250mm I could find any cheaper was this $US450 Tamron, but there's no mention of stabilisation. I spent a week or so taking all kinds of pictures...
The trouble comes when you start doing math. If you bought an entry-level DSLR camera from Canon, Nikon, Sony or Pentax—all compatible with the appropriate models of this lens—chances are you already have a lens, more than likely something in the 18-55mm range. That means that the chances of you being able to pick up a complementary lens in the 55-200mm or even 55-250mm range for under $US200 are high. Suddenly, you're outfitted exactly like you would be if you bought this 18-250mm lens, and you'd probably have saved money doing it.
And as I said, it's not a fast lens, and many would probably recommend something with more performance. If you're going to buy multiple lenses, why not buy ones that are dedicated to particular tasks. I call your attention to this Listmania list on Amazon, which shows nice budget-minded Canon lens selections. If you're looking at the used pricing (which most people recommend is the way to go if you're not a pro), you can see that $US530 would get you two, maybe three really nice fast lenses to augment your kit lens, in assorted focal ranges.
I'm not going to bash Sigma—I'm not pretending to be experienced enough to know a great lens from a crappy one of equal spec, but my time with this lens—paired to a Canon T1i—has been fruitful, especially out of doors and in locations where a little extra zoom could reduce the need for cropping and the inevitable lack of crispness that comes as a result. A pro photographer buddy I was talking to recently said that Sigma was a reliable maker of cheap lenses to go with Nikon, Canon and other DSLRs. He said among his collection of about 12 lenses, a few were Sigmas.
In the end, I have to face down my own laziness: The smartest thing to do is not to buy the single 18-250mm lens and hope for some decent light. No, the smartest thing to do is put money into multiple lenses, Sigmas or otherwise,
Now, if you more experienced shooters want to share your knowledge of Sigma, Tamron, any of the big camera makers, or the wisdom of buying lenses used, this here's your chance. Please, hit the comments below. [Sigma]
Excellent single lens for covering range from 18mm to 250mm in an entry-level DSLR from Canon, Nikon, Pentax or Sony
Some camera kits already come with two lenses that cover 18-55mm and 55-200mm, or one that goes from 18-105mm or 18-200mm, making most of this lens redundant
Even though image stabiliser does allow for improved shooting in low light or at extreme zoom, there are better specialty lenses at each particular focal length, possibly cheaper when bought used